Hydraulic fracturing is an important means for oil and gas exploitation in unconventional oil and gas reservoirs. The core idea is to rapidly increase the pressure in formation fracture by injecting a large displacement volume of high-pressure fluid into the fracture, causing the fracture to rapture and extend along the tip. However, at the same time, the high-pressure fluid will also produce fluid loss along wall surfaces of the fracture. According to the principle of force and reaction force, the fluid loss will generate an additional force near the fracture wall surfaces under higher pressure gradient which changes as the fracture surface distance changes, thus changing the original effective stress of the fractured formation. This additional force is referred to as seepage force.
At present, the concept of seepage force is only applied to soil mechanics. It is used to study the pushing, rubbing and dragging effects of fluid on soil particles along the direction of fluid flow upon seepage of the fluid through soil, which are combined to form forces acting on soil skeleton. Existing seepage force measuring devices and methods are based on soil particles. Due to the loose structure of soil particles, the effective stress change perpendicular to the fluid flow direction does not need to be considered, and the existing devices and methods are not suitable for measuring seepage force generated by hard fractured formations under the action of high-pressure fluid, making the test measurement and calculation methods unable to meet the requirements for the study of fractured formations.
In the current hydraulic fracturing test, the pore pressure in the sample is a fixed value, and there is no device or method for measuring the magnitude of the seepage force near the fracture wall surface and the effective stress of the fractured formation caused by the seepage force. In view of the fact that the seepage force measurement methods in the prior art do not take the hard fractured formation into consideration, and the hydraulic fracturing test does not study the seepage force, it cannot truly reflect the change of effective stress near the fracture wall surface during hydraulic fracturing. Therefore, a new measuring device and method are proposed to calculate the magnitude of a seepage force generated under high injection pressure and the change of effective stress of the fractured formation caused thereby.